This invention relates to low fusing high-expansion dental porcelain especially useful for the fabrication of both all-ceramic and porcelain-fused-to-metal (PFM) restorations.
Porcelains are typically designed to be used in the manufacture of either all-ceramic dental restorations or in PFM restorations, but are not normally functional with both types of restorations due to the differences in properties of ceramics and metals. One such porcelain, OPC(copyright) Low Wear(trademark) porcelain, available from Jeneric/Pentron Inc., Wallingford, Conn. and covered in copending, commonly assigned patent application Ser. No. 09/133,582 filed Aug. 13, 1998, now U.S. Pat. No. 6,120,591, which is hereby incorporated by reference, was initially designed to be used as overlay for pressed all-ceramic restorations as well as for the fabrication of porcelain jacket crowns and veneers. However, OPC(copyright)Low Wear(trademark) porcelain is not currently used for PFM restorations despite its wear resistance, forgiveness to natural dentition and strength being superior to those of conventional PFM porcelains as shown in the Table 1 below:
The major obstacle preventing use of the OPC(copyright) Low Wear(trademark) porcelain in PFM restorations is the absence of an opaque/alloy combination compatible with this porcelain having relatively high expansion of about 17xc3x9710xe2x88x926/xc2x0 C. (25xc2x0 C.-500xc2x0 C.).
There exists a Golden Gate System(trademark) for PFM restorations available through Degussa(trademark) (Dental Division, South Plainfield, N.J.) which combines Duceragold(trademark) porcelain and Degunorn(trademark), type IV crown and bridge alloy (CTE=16.4xc3x9710xe2x88x926/xc2x0 C., 25xc2x0 C.-500xc2x0 C.). This system requires rather tedious multistep alloy preparation procedures including a necessary wash bake step prior to application of the opaque; and an excessively long (16-20 min) and complex first dentine bake to assure proper bonding and compatibility of the Duceragold(trademark) porcelain to the Degunorm(trademark) alloy. In particular, the cooling segment (3-4 min between 720xc2x0 C. and 680xc2x0 C.) in the first dentine bake is required by the manufacturer to grow additional leucite and may be an indication of instability of leucite in this porcelain. The following Table 2 below sets forth the various properties of the Duceragold(trademark) porcelain.
U.S. Pat. No. 5,453,290 to Van der Zel is directed to a dental porcelain for use with a dental alloy. The porcelain described therein must be fabricated from three frits, making it more difficult and costly to control the expansion and the glass transition temperature of the final product. Moreover, the CTE of the porcelain must be below the CTE of the alloy by 0.5-1.5 limiting the components to be used together. There is a need to provide a porcelain-fused-to-metal system for dental restorations having simple manufacturing procedures. It is desirable to provide a porcelain that is compatible with alloys of relatively high expansion. It is desireable to provide a two-frit porcelain for use in PFM restorations.
These and other objects and advantages are accomplished by opaque porcelains for use with metal cores in the manufacture of PFM restorations. The porcelains exhibit a coefficient of thermal expansion (CTE) substantially equal to or slightly above the CTE of the metal to which it is applied. In a preferred embodiment, the porcelains exhibit a CTE equal to or up to about 1.0xc3x9710xe2x88x926/xc2x0 C. higher than the dental alloys to which they are applied as the opaque. The porcelains are fabricated from a mixture of two frit compositions. A high expansion, leucite containing frit is combined with a low fusing glass frit to provide a porcelain having an expansion in the range of 16.9 to about 18.5xc3x9710xe2x88x926/xc2x0 C. in the temperature range of 25xc2x0-500xc2x0 C. By combining two frits, the expansion and fusing temperature can be controlled to the values stated above.
The invention relates to a porcelain material for use in all-ceramic restorations and PFM restorations. The porcelains exhibit a coefficient of thermal expansion (CTE) substantially equal to or slightly above the CTE of the metal to which it is applied. Preferably, the porcelains exhibit a CTE equal to or up to about 1.5xc3x9710xe2x88x926/xc2x0 C. higher than the dental alloys to which they are applied as the opaque, and more preferably equal to or up to about 1.0xc3x9710xe2x88x926/xc2x0 C. The porcelains are compatible with metals having coefficients of thermal expansion (CTE) in the range of from about 15.5 to about 17xc3x9710xe2x88x926/xc2x0 C. in the temperature range from 20xc2x0 to 500xc2x0 C. The porcelains exhibit CTEs in the range of about 16.9 to about 18.5 in the temperature range from 20xc2x0 to 500xc2x0 C., and preferably in the range of about 17 to about 17.5xc3x9710xe2x88x926/xc2x0 C. in the temperature range from 20xc2x0 to 500xc2x0 C. The porcelains are fabricated from a mixture of two frit compositions. A high expansion, leucite containing frit is combined with a low fusing glass frit to provide a porcelain having an expansion in the range of 16.9 to about 18.5xc3x9710xe2x88x926/xc2x0 C. in the temperature range of 25xc2x0-500xc2x0 C. It is essential to this invention that both the high expansion and the low fusing components of the two-frit mixture exhibit a low glass transition temperature (GTT). It is extremely important that the low fusing component used in the opaque formulation exhibits a GTT lower than about 415xc2x0 C. By combining two frits, the expansion and firing temperature can be controlled to the values stated above. Opaque porcelains herein having pigments exhibit a coefficient of thermal expansion of average value of about 17xc3x9710xe2x88x926/xc2x0 C. in the temperature range of 25xc2x0-500xc2x0 C. Opaque porcelains without pigments, i.e., white opaques, exhibit coefficients of thermal expansion in the higher end of the range, such as about 17.5 to about 18.5xc3x9710xe2x88x926/xc2x0 C. in the temperature range of 25xc2x0-500xc2x0 C.
Table 3 below shows the compositional ranges of the porcelains for use in the invention.
As set forth in Table 3 above, Li2O, present in an amount of from about 1.5% to about 3%, and F, present in an amount of 0.4%-1%, are instrumental in providing a low glass transition temperature. The presence of Li2O and F also assist as well in increasing the coefficient of thermal expansion and decreasing the maturing (firing) temperature. The high expansion, leucite containing component of the two-frit mixture has a reasonably low glass transition temperature as well. This is achieved by maintaining a reasonably low molar ratio of Al2O3 to the sum of alkali and alkaline earth oxides (R2O+RO). Normally, these compositions are extremely unstable and reactive as well as prone to sanidine precipitation in the temperature range of 650xc2x0 C.-950xc2x0 C. It was surprisingly found that certain compositions with specific combinations of K2O, Na2O and Li2O are very stable. In addition, it was found that the molar ratio of Al2O3/K2O should be within the range of 0.73-0.95 to assure both the required thermal stability and low glass transition temperature of the high expansion component of the porcelain. The low glass transition temperature provides a porcelain having good compatability with alloys having CTEs in the range of about 15.5 to about 17xc3x9710xe2x88x926/xc2x0 C. in the temperature range of 25xc2x0-500xc2x0 C. Table 4 below sets forth the properties of the porcelain compositions.
Table 5 sets forth compatible alloys for use with the porcelains.
Table 6 below shows compositional examples of body (incisal) and opaque porcelains.
The low glass transition temperature of the opaque porcelain is paramount to assure its compatibility with alloys having CTE""s in the range of about 15.5 to about 17, such as commercially available Gold Core 75(trademark) alloy from Jeneric/Pentron Inc., Wallingford, Conn. This is a gold alloy that does not contain copper and other elements that form dark oxide layers and, therefore, requires much less intricate preparation procedures compared to the Degunorm alloy. Specifically, the Gold Core 75(trademark) alloy forms an adequate oxide layer without compromising the appearance of the coping when degassed at 870xc2x0 C.-885xc2x0 C. for about 5-7 min in air or vacuum.
The high potassium content in the porcelain is essential to assure high stability of leucite. High potassium oxide content combined synergetically with other alkali elements (Li and Na) assures as well relatively low glass transition temperature (GTT) and, hence, increased resistance to thermal expansion mismatch cracking and increased adaptability to alloys of slightly lower expansion. It was surprisingly found that increased potassium content increases stability in compositions with low GTT, e.g., Example 1 was found to be much more stable than Comparative Example 1 (compare K2O content). Specifically, dental porcelain of Example 1 has excellent thermal expansion stability and maintains the same thermal expansion after 5 successive bakes at its firing temperature. Dental porcelain of Comparative Example 1 was found to change thermal expansion and opacity upon multiple bakes.
Essential to this invention is that opaque compositions possess a relatively low transition temperature and contain the same elements such as ZnO and Ta2O5 as the oxide layer forming on the alloy that assure good bonding to alloys. Specifically, the oxide layer on the Gold Core 75(trademark) alloy was found to be enriched with ZnO and Ta2O5 and the same components were included in the opaque formulation to improve bonding.
Body and incisal porcelain are typically applied to opaque porcelain, respectively. Preferably, the body and incisal porcelains used with the opaque herein exhibit an average coefficient of thermal expansion of about 17.2.
In a preferred embodiment of the invention, alloys having a CTE in the range of about 15.5 to about 17xc3x9710xe2x88x926/xc2x0 C. in the temperature range of 25xc2x0-500xc2x0 C. are used to manufacture a metal core for a restoration. Opaque porcelains are applied thereto, wherein the CTE is in the range of about 16.9 to about 17.5xc3x9710xe2x88x926/xc2x0 C. in the temperature range of 25xc2x0-500xc2x0 C. and body porcelains are applied thereto having CTEs in the range of about 16.9 to about 17.7. It is preferred that the opaque porcelain has a CTE about equal to or up to about 1.5xc3x9710xe2x88x926/xc2x0 C. higher than the metal core. It is preferred that the body porcelain has a CTE about equal to or up to about 1.5xc3x9710xe2x88x926/xc2x0 C. higher than the metal core. It is preferable that the CTE of the opaque is between the CTE of the alloy and the CTE of the body porcelain.